The invention relates to a polarimeter and method for measuring the rotation of optically active samples
Generally, polarimetric analysis is a method of chemical analysis based on the optical activity of the substance to be determined, i.e., a measurement of optical rotation of the substance is used to identify the substance or determine quantity. A polarimeter is an instrument used to determine the rotation of the plane of polarization of plane polarized light when it passes through a substance.
In German patent application 1 159 180 (which is incorporated herein by reference), a polarimeter with a polarizer, an analyzer and a radiation receiver arranged behind the analyzer is disclosed in which an optically active sample is inserted between the polarizer and the analyzer, and the polarizer is caused to make rotary oscillations about the beam axis. The output signal of the radiation receiver is supplied to phase sensitive balancing means which, when the polarization plane rotates, effect a follow-up rotation of the analyzer.
In the prior art polarimeter, the polarizer is connected to a cylindrical, diametrically magnetized permanent magnet. The permanent magnet is surrounded by concave-cylindrical pole surfaces of a electromagnet exited by an A.C. voltage. A radial leaf spring fetters the permanent magnet and the polarizer to its rest position.
In the prior art driving device, the polarizer oscillates sinusoidally with the mains frequency near the resonance of the oscillatory system formed by the polarizer, permanent magnet, and leaf spring. For balancing, the analyzer is rotated by a servomotor depending on the signal of the detector as long as the analyzer is arranged crosswise with the rotated polarization plane of the measuring light beam. The oscillation of the polarizer provides an alternating signal with an oscillation frequency according to which the servomotor can be controlled.
It is necessary to measure the optical rotation of the measuring light beam with a very high degree of precision (millidegree). Therefore, the analyzer has to follow-up the polarization plane of the measuring light beam very sensitively with a large stepping-down ratio. On the other hand, quite large optical rotations, e.g. 90.degree. rotations, can occur and considerable time, relatively speaking, is required until the analyzer with the correspondingly large stepping-down ratio is rotated to the balancing position.
Accordingly, it is an object of the present invention to provide a new and improved polarimeter which attains a high degree of precision in angular measurement with decreased measuring time.
It has been found that the foregoing and related objects are attained in a polarimeter having a light source generating a measuring light beam, a polarizer mounted for rotation about the beam axis of the measuring light beam, a driving mechanism for reciprocatingly rotational movement of the polarizer, a sample vessel, an analyzer behind the sample vessel, a photoelectric detector sensing the measuring light beam, and balancing apparatus controlled by the signal of the photoelectric detector for positioning the polarizer and analyzer substantially crosswise. The balancing apparatus comprises a stepper motor for rotating the polarizer with an adjusting movement superimposed by a reciprocatingly rotational movement. By the adjusting movement, a quick coarse balancing of the polarizer plane of the measuring light beam relative to a zero position of the analyzer is achieved. Fine measuring means are provided for fine measurement of the angle between the polarization plane of the analyzer in its zero position and the polarization plane of the coarsely balanced measuring light beam.
Thus, the polarizer is utilized not only for generating a reciprocatingly rotational movement of the polarization plane but also for a coarse balancing movement by which the polarization plane of the measuring light beam, which becomes effective at the analyzer with consideration of the optical rotation in the sample, is at least approximately moved into a position in which the polarization plane of the substantially stationary analyzer is arranged crosswise. Thereby, the polarizer makes a movement which is composed of a steady adjusting movement and a reciprocating rotational oscillation movement superimposed onto this adjusting movement. Such coarse balancing can be achieved relatively quickly and fine measuring of the angle is accomplished by separate fine measuring means.
The fine measuring means can measure the exact angle by interpolation of the alternating signal for right-hand and left-hand rotation remaining after the coarse balancing. The fine measuring can also be made by a fine balancing in which the analyzer is rotated. Since the analyzer has to be rotated only through a small angle after the coarse balancing, this rotation can be made quite quickly in spite of the required stepping down ratio.